Dry electrode cap for electro-encephalography

ABSTRACT

An electro-encephalography electrode cap for contacting the scalp of a head with a number of pin-shaped electrodes for contacting the scalp, and an electrode holding device. The electrodes are mounted on the electrode-holding device through at least one elastic joint.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U. S. national phase of PCT/EP2006/011843 filedDec. 8, 2006. The disclosure of PCT/EP2006/011843 is hereby incorporatedherein by reference.

The invention relates to an electrode cap for electro-encephalographywhich allows contacting the head of a human or an animal without aconductive gel between the scalp and the electrode itself.

The most common non-invasive solution to collect electro-encephalography(EEG) data, applied in biomedical research and hospitals, is the wetelectrode, comprising a metal plate coated with Ag. This wet electroderequires gel to be applied between the skin and the electrode to allowfor the exchange of ions at the interface. The use of such a gel isinconvenient for a daily use of the electrodes: it requires a timeconsuming preparation before any EEG recording can be carried out; thegel dries with the passing of time and thus needs to be refilled in longapplications of the electrodes to the scalp. It has also been reportedthat long term contact with the skin produces irritation.

An alternative approach to the collection of EEG data is the use ofsensors that operate with current displacement, that is a capacitivetransduction, instead of charge current, as was the case for theprevious example. These types of electrodes do not require physicalcontact to the signal source. However, the advantage of independence ofcontact with the body brings along the disadvantage that the backgroundnoise recorded along with the physiological signals is higher.

All other solutions make contact with the skin. Some make use of NASICONceramic material. The particular propriety of interest of the NASICON(acronym for Na Super Ionic Conductor) material is its very highconductivity of Na⁺ ions even at room temperature. The downside of usingthis material is a higher impedance mismatch with the skin than thecommon wet electrodes.

Microfabrication technology has produced good results in miniaturizationof real size sensors. As interesting examples, Griss et al. developed adry electrode that avoids the use of electrolytic gel and at the sametime fixes the electrode to the skull surface with enough reliability toavoid motion artifacts, making use of microfabricated spikes on theelectrode surface (Griss, P et al., “Characterization of micromachinedspiked biopotential electrodes”, Biomedical Engineering, IEEETransactions on Volume 49, Issue: 6 Jun. 2002).

There is a higher potential difference measured between a pair of suchelectrodes than in the case of the standard wet electrodes which couldbe due to the influence of the potential of the sweating duct membranesin the dermis. Despite this fact, it seems that sweat on the skin doesnot produce so much variation on the electrode impedance as occurs withAg/AgCl electrodes. The spikes prickle the outer layers of the skin butavoid penetrating the dermis, where nerves and blood vessels are. Thisway, pain-free measurements of potentials is achieved, avoiding the highimpedance from the outer skin layers.

Another solution makes use of carbon nanotubes. In a similarconfiguration, a population of carbon nano-tubes are used as probesthat, in pricking the surface layers of the dermis, behave as thetransducers themselves.

It is therefore the problem to be solved by the present invention toprovide an easily manufacturable, affordable device which can be placedon the head in a few minutes, is comfortable, makes reliable long-termskin contact without pain, and provides enough accuracy in recording ofbrain activity for applications such as for example brain-computerinterfaces.

The present invention is based on the idea that an optimal result of anelectro-encephalography measurement (EEG) together with a maximum ofwearing comfort for the patient is achieved if the force that each pinexerts on the scalp of the patient is uniform for all electrodes orgroups of electrodes and can be adjusted.

According to the present invention, this is achieved by mounting orsupporting the electrodes on an electrode holding means through at leastone elastic joint. The electrodes push down onto the surface of thescalp by means of an elastic force, as for example that of a spring orof pneumatic pressure.

The electrode holding means preferably comprises a head-fixing means,such as for example an adjustable head strap which tightens around theforehead, side and back of the head and allows to fix the electrode capto the head with adjustable pressure.

Preferably, the electrodes are mounted on the electrode holding meansthrough one or more connecting means which preferably are or comprise asmall number, for example 2 to 30, holding arms. As many arms can beused as can be fit without interfering with each other physically. Thoseholding arms are fixed to the head fixing means or head strap with oneend while the other end carries the electrodes. The electrodes can beattached to the holding arms directly or indirectly through othercomponents.

It is preferred that the holding arms comprise two legs which areconnected with each other through an elastic joint. The lower part ofthe arm, that is the leg which is connected to the head fixing means, iscalled electrode arm support. To this an electrode arm moving beam, thatis the leg which carries the electrodes, is attached by means of arevolute or prismatic joint. Preferably, this allows the distal end ofthe arm to move perpendicularly to the surface of the scalp. If the legsof the arms are straight they preferably include an angle which opens inthe direction of the head. The elastic joint between the legs of an armcan be a revolute joint or a pivotal joint of which the rotation axis isperpendicular to the length of the legs and tangential to the head.

It is preferred that a torque-producing flexing cord or a similarlyadjustable torsional spring is stretched between pairs of adjacent legsof an arm which are connected with each other by an elastic joint. Thisallows to apply a bending moment around this elastic joint whichconnects the two legs. By adjusting the length of such a cord or spring,the pressure which the electrodes apply to the scalp of the head can beadjusted. The cord or spring may have a certain elastic stiffnessK_(EAFC) and zero point X_(0EAFC) either of which may be adjustable byhand via a tightening screw or a geared mechanism.

It is also possible that the joint between two adjacent legs is aprismatic joint which is preferably movable in the direction of the headof the patient. Such a prismatic joint comprises a bore or cylinder inone leg in which a part of the other leg is guided in one direction, forexample the direction of the length of the leg.

Between the two legs, a spring or some other flexible element can bearranged, which expands or contracts if the legs are moved against eachother. It is furthermore possible to connect two neighbouring legs of anarm by a flexing cord, similarly as outlined for the pivotal joint,above. By this a force is applicable in the direction in which theprismatic joint is movable. This force can act in the direction of theelastic force of the elastic element or in the opposite direction. Byadjusting the length of such a cord, the pressure which the electrodesapply to the scalp of the head can be adjusted. This cord also may havea certain elastic stiffness K_(EAFC) and zero point X_(0EAFC) either ofwhich may be adjustable by hand via a tightening screw or a gearedmechanism.

Preferably, the individual electrodes are grouped into groups wherebyeach group comprises a part of the electrodes. The number of electrodesin each group is preferably equal but can also be different.

Electrodes which belong to the same group are held by a common groupholding means which holds all electrodes of this group. The groupholding means are each mounted directly or indirectly on the electrodeholding means through at least one elastic joint. As explained abovethis holding means can be an electrode holding arm wherein the differentlegs are connected through elastic joints.

The group holding means preferably comprise at least one elastic jointthrough which they are mounted on the electrode holding means or theholding arm. Such an elastic joint can be a semi-rigid spherical jointor a virtual ball joint which is an elastic structure which, through itselasticity, provides the same motion as a ball joint on the distal endof a leg attached to the joint. Its response to deflections isequivalent to that of a spring-loaded ball joint. Such a joint can bemovable around a first axis parallel to the length of the electrodes,i.e. parallel to the direction of the force by which the electrodes arepressed against the head. For this direction, the joint then has acertain torsional stiffness K_(S). It is furthermore possible that thejoint is movable around at least one axis perpendicular to the directionin which the force acts which applies pressure to the head. In thisdirection, the joint has a torsional stiffness K_(T). If the joint ismovable around an axis perpendicular to the length of the electrodes,the surface which is described by the tips of the electrodes can adjustits orientation to the slope of the head at the position where theelectrodes are applied. It is preferred that the torsional stiffnessaround the axis perpendicular to the length of the electrodes K_(T) isconsiderably greater than the torsional stiffness K_(S) around the axisparallel to the length of the electrodes. By this, the group holdingmeans can be prevented from rotating excessively around a directionnormal to the scalp surface.

Preferably, the electrodes of a given group are grouped into sub-groupsor bundles which each contain the same number of electrodes or a similarnumber of electrodes with parallel lengths. The electrodes areperpendicular to the surface on which they are located. There may existtwo, three or more bundles of electrodes in one group. Within a bundle,the electrodes can be arranged in a shape which has a circular,elliptical, triangular or rectangular outline. The electrodes within abundle can be arranged in concentrical circuits or some electrodes canbe arranged around a center electrode. However, also other arrangementsof the electrodes within a bundle are possible. If necessary, theelectrodes can be arranged in bundle holding means which may be arrangedat the group holding means through a joint which may be an elasticjoint. If the electrodes are bundled the contact to the skin is ensureddespite hair and surface irregularities. Each additional pin adds morepotential contact surface between metal and skin, thus lowering theeffective electrode impedance.

A group holding means can comprise two, three or more bundles ofelectrodes. If there are three bundles, those can be located at thecorners of a triangle, preferably an equilateral triangle. In thisconfiguration, the group holding means can have a Y-structure, i.e. astructure with three straight legs which meet with one end at one pointand preferably have the same length. The angles between the legs arepreferably all equal.

An Y-shaped group holding means allows each branch of the Y to makecontact with the scalp independently, as the spherical joint allows.

The relative stiffness among electrodes in a bundle is high, while thestiffness of the moving beam of the entire bundle's electrode arm isrelatively low.

If the bundles have three electrodes, those can also be placed at thecorners of a triangle, which is preferably an equilateral triangle. Thecentre of this triangle marks the location of the bundle.

It is preferred that each single electrode is elastically supported.Hereby, the electrodes can be guided in a guide member which guides theelectrodes in the axial direction of their length, that is basically inthe direction of the scalp. Within the guides the electrodes aresupported on an elastic element which is elastic in the direction inwhich the electrodes can move in the guide. The electrode is thusarranged like a piston in a cylinder. The elastic element can be aspring which is placed inside the guide member behind the electrode indirection of the axial length of the electrode.

Alternatively, the electrodes can be virtually compressible, i.e. theyare elastically deformable. Here the electrodes can be thin metal pins.

It is preferred that the electrodes are coated or plated in a highconductance material, such as for example gold, platinum, silver, silverchloride other precious metals, alloys and/or conductive nanoparticles.

The electrodes are intended to measure electrical signals in anelectro-encephalography. For this purpose, the electrodes can be wiredin unipolar configuration in which all pins in a bundle are in contactwith each other. A voltage can then be measured with reference toground. Alternatively, the electrodes can be arranged in a bipolarconfiguration where the electrodes of a bundle, a group or allelectrodes are separated into two groups, wherein the electrodes of onegroup are electrically connected with each other and the electrodes ofthe other group are connected electrically with each other so that avoltage between the electrodes of the two groups can be measured.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an electrode cap according to the present invention mountedon the head of a patient.

FIG. 2 shows a head strap with a holding arm which comprises a pivotaljoint.

FIG. 3 shows a head strap with a holding arm which comprises a prismaticjoint.

FIG. 4 shows a group holding means with three bundles of electrodes.

FIG. 5 shows a group holding means which is deformable around an axisparallel to the length of the electrodes.

FIG. 6 shows an electrode cap mounted on the head of a patient viewedfrom above.

FIG. 7 shows three elastically supported electrodes.

FIG. 8 shows virtually compressible electrodes mounted on a groupholding means.

FIG. 9 shows a unipolar configuration of an electrode bundle.

FIG. 10 shows a bipolar configuration of the electrodes in a bundle.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrode cap according to the present invention mountedon the head 2 of a person. The electrode cap comprises a head strap 1which runs around the head 2. On opposite sides of the head strap 1, twoholding arms 3 a and 3 b are attached. The holding arms 3 a and 3 b eachcomprise two legs 4 a and 5 a as well as 4 b and 5 b which are connectedwith each other through revolute joints 6 a, 6 b. The lower legs 4 a and4 b, which are supporting legs, are attached to the head strap 1 withone side and to the joints 6 a and 6 b, respectively, with the otherside. The second legs 5 a and 5 b, which are electrode arm moving beams,carry group holding means 7 a, 7 b. Those have the form of3-branch-trees on which three bundles of electrodes are accommodated atthe ends of its legs.

The electrode arm support 4 a, 4 b and the electrode arm moving beam 5a, 5 b are each arranged at an angle which opens in the direction of thehead. Between the two legs 4 a and 5 a as well as 4 b and 5 b, a flexingcord 8 a, 8 b is stretched. By these flexing cords 8 a, 8 b, thepressure by which the electrodes are pushed on the head 2 is adjustable.

FIG. 2 shows the electrode arm 3 a in detail. The electrode arm 3 a ismounted on the head strap 1. It comprises a first leg 4 a and a secondleg 5 a which are connected with each other through a pivotal joint 6 a.Between the electrode arm support 4 a and the electrode arm moving beam5 a, a flexing cord 8 a is stretched which bends the electrode arm 3 aaround the joint 6 a. The angle between the legs 4 a and 5 a isadjustable by changing the tension of the string 8 a. The flexing cord 8a may be elastic itself with an elastic stiffness K_(EAFC). The tensionof the cord 8 a can be adjusted by hand with the tightening screw 9.

FIG. 3 shows an alternative construction of the arm 3 which correspondsto the arms 3 a and 3 b in the previous figures. The first leg 4 a,which is the electrode arm support, is connected with the second leg 5 avia a prismatic joint 10. The prismatic joint 10 is given by a drillingin the first arm 4 a in which the second arm 5 a is guided in thedirection of its length.

Again, the legs 4 a and 5 a build an angle which opens in the directionof the head. Also the electrode arm support may have an angle, openingin the direction of the head. The second arm 5 a carries a group holdingmeans 7 through a semi-rigid spherical joint 13. The group holding means7 carries the electrodes 12. Between the group holding means 7 and thefirst arm 4 a, a spring 11 is located parallely to the second arm 5 a,preferably surrounding the second arm 5 a. This spring 11 thereforecontracts or expands if a force is applied on the second arm 5 a in thedirection of its length. This happens for example when the electrodes 12are pressed against the head of a patient.

FIG. 4 shows a group holding means carrying three bundles of electrodes12. Each bundle comprises three electrodes 12 which are located at thecorners of an triangle, for example an equilateral triangle. The upperpart of the figure shows the view perpendicular to the length of theelectrodes while the lower part of the figure shows a view from above.It can be seen from the lower part of the figure, that the group holdingmeans 7 has a Y-structure with three legs 14 a, 14 b, 14 c of equallength which are arranged in equal angles to each other. The holdingstructure 7 has a semi-rigid spherical joint 13 through which it isconnected with the holding arm 3.

The semi-rigid spherical joint 13 is elastically movable around an axis16 which is parallel to the electrodes 12 as well as one or two axises15 a and 15 b which are perpendicular to the direction of the electrodes12. The torsional stiffness K_(T) of the first axis is considerablygreater than that of the second axis K_(S), in a similar manner astorques produced in response to motion by an elastic U-joint.

FIG. 5 shows an alternative construction of the group holding means 7.Here, the holding means 7 is fixed at a virtual ball joint 17. A virtualball joint is an elastic structure which, through its elasticity,provides the same motion as a ball joint on the distal end of an legattached to the joint. Its response to deflections is equivalent to thatof a spring-loaded ball joint.

The element 17 is bendable around at least one axis perpendicular to theelectrodes 12 and may also be flexible around an axis parallel to theelectrodes 12.

FIG. 6 shows an electrode cap according to the present invention viewedfrom above the head of the patient. The shown configuration is a samplefor BCI applications that utilize motor imagery. The head strap 1 runsaround the head of the patient and is closed with a closing means 18.This may allow the adjustment of the head strap 1. The electrode arms 3a and 3 b carry the group holding means 7 a and 7 b at which theelectrodes 12 are arranged. In the shown example, the group holdingmeans 7 a and 7 b have the Y-structure as described above. Theconfiguration allows access to the central and lateral regions of thescalp and therefore the brain.

FIG. 7 shows electrodes which are elastically supported along their axisby means of moving parts and elastic elements. The electrodes 12 a, 12 band 12 c are located in a holding means 7 and are each located in guidemembers 19 a, 19 b and 19 c which only allow the electrodes to move inthe direction of their axial length. The electrodes are supported incontact with elastic elements, as for example springs 20 a, 20 b and 20c, which are fixed at the guiding means 19 a, 19 b, 19 c with one endand are in contact with the electrodes 12 a, 12 b and 12 c with theother end, respectively. The electrodes 12 a, 12 b and 12 c cantherefore move in the cylindrical tubes 19 a, 19 b and 19 c like apiston in a cylinder. The example only shows three electrodes, however,any number of electrodes can be placed in a bundle, a group or theelectrode cap.

FIG. 8 shows virtually compressible electrodes as an alternative forelastically supported electrodes. The left part of FIG. 8 shows thoseelectrodes 12 a, 12 b, 12 c mounted on a holding means 7 if no force isapplied to the electrodes. The right side of FIG. 8 shows the same setupif force is applied to the electrodes 12 a to 12 c. Here, the electrodesare elastical themselves, i.e. they bend elastically when a force isapplied. In other words, the electrodes can deflect in similar means asabove by the flexibility of the shape and material from which they aremade.

FIG. 9 shows a unipolar configuration of electrodes in a bundle. Sixelectrodes 12 a to 12 f are grouped around three electrodes 12 g to 12i. All electrodes are electrically connected with each other and avoltage V_(u) is measurable with reference to ground.

FIG. 10 shows a bipolar configuration of electrodes in a bundle. Theelectrodes are grouped into two parts, which form the two poles betweenwhich the voltage V_(b) is measurable. The electrodes 12 a and 12 bbelong to one pole while the electrodes 12 c and 12 d belong to theother pole. The electrodes of each part are electrically connected witheach other.

The electrode cap according to the present invention is applicablewherever electro-encephalography recordings are desirable with minimalpreparation and long duration, that is duration longer than, e.g. twohours. Those are for example medical diagnosis and monitoring,brain-computer-interfaces (BCI), lie detection or monitoring of userattention in safety-critical operation of machines. The electrode capaccording to the present invention does not require conductive gelbetween the scalp and the electrodes and the force that each electrodepin exerts on the scalp is uniform and does not cause pain to thepatient while the electrodes are in stable contact with the skin. Theoverall level of force or the force applied by certain groups ofelectrodes is adjustable.

The invention claimed is:
 1. An electrode cap for contacting the scalpof a head in an electro-encephalography with a number of pin-shapedelectrodes for contacting the scalp, and an electrode holding meanswherein the electrodes are mounted on the electrode-holding meansthrough at least one first elastic joint, the electrodes being groupedinto at least one group, the electrodes of each group being mounted on acommon group-holding means and the group-holding means each beingmounted on the electrode-holding means through at least one secondelastic joint through which the electrodes comprising the group aremounted on the electrode-holding means.
 2. The electrode cap accordingto claim 1 wherein the second elastic joint is at least one of asemi-rigid spherical joint and a virtual ball joint which is at leastone of movable around a first axis parallel to the length of theelectrodes of this group with a torsional stiffness K_(s) and movablearound at least one axis perpendicular to the length of the electrodeswith a torsional stiffness of K_(T).
 3. The electrode cap according toclaim 2 wherein K_(T) is greater than K_(S).
 4. An electrode cap forcontacting the scalp of a head in an electro-encephalography with anumber of pin-shaped electrodes for contacting the scalp, and anelectrode holding means wherein the electrodes are mounted on theelectrode-holding means through at least one first elastic joint, theelectrodes being grouped into at least one group, the electrodes of eachgroup being mounted on a common group-holding means and thegroup-holding means each being mounted on the electrode-holding meansthrough at least one second elastic joint, the electrodes of at leastone group being grouped into three bundles which are located at thecorners of a triangle.
 5. The electrode cap according to claim 4 whereinthe group-holding means has a Y-structure, whereby the three bundles arelocated at the ends of the legs of the Y-structure.
 6. An electrode capfor contacting the scalp of a head in an electro-encephalography with anumber of pin-shaped electrodes for contacting the scalp, and anelectrode holding means wherein the electrodes are mounted on theelectrode-holding means through at least one first elastic joint, theelectrodes being grouped into at least one group, the electrodes of eachgroup being mounted on a common group-holding means and thegroup-holding means each being mounted on the electrode-holding meansthrough at least one second elastic joint, the electrodes of at leastone group being grouped into at least two bundles, each bundle havingthree electrodes which are located at the corners of a triangle.